If we're comparing story and all those stuff, Steven Universe wins. I saw Adventure Time premier back then and fell in love with it but the its current plot is okay compared to the first few seasons. The thing is most of my favourite arcs in Adventure Time like Marceline and Simon is written by Rebecca Sugar( creator of SU). She also wrote my favourites songs in the series. But when she left Adventure Time, the show lost it's magic I think. It's still good but not as good as when she was still there. Then Steven Universe came. Honestly, in the beginning, I didn't like it at all. Until the "Giant Woman"( s1, eps 12 I think). I fell in love with the concept of fusion and as the plot prodressed, it became better and better. Ok Ko on the other hand is just starting out so I'm not including it.